In copending patent application Ser. No. 431,020, filed Jan. 7, 1974, which is a continuation of Serial No. 158,172, filed June 30, 1971, now abandoned there is disclosed a process known as ionography which utilizes a photoemissive layer disposed on a suitable conductive substrate spaced from a second conductive layer having an insulative receptor material in the form of a sheet or the like disposed thereon. A d.c. voltage is applied across the gap between the conductive plates such that one is positive and one is negative. The arrangement can be utilized to make X-ray images of an object placed adjacent one of the plates between that plate and an X-ray source. The photoemissive layer is preferably an efficient absorber of X-rays. In the early work in the field of ionography, such as that disclosed by K. H. Reiss in Z. Angew, Physik, Volume 19, page 1 (1965), a heavy metal such as lead was utilized as a photoemitter. A quenching gas is flowed or may be stationary within the gap between the plate electrodes. When an object is disposed adjacent the anode and is irradiated by X-rays or gamma rays, the electromagnetic radiation is differentially absorbed by the object and passes through the transmissive anode and insulative layer affixed thereto, and across the gap to strike the photoemitter where it is strongly absorbed by the photoemitter. As a consequence, the photoemitter ejects electrons having energies up to many kilo-electron volts. The number of electrons emitted is dependent upon the number of X-ray photons absorbed in that portion, the depth of absorption and the photon energy. On leaving the photoemitter surface, the electrons find themselves in the d.c. field between the electrodes and travel toward the positive electrode. The quenching gas serves to slow down the electrons so that they will not scatter when reaching the insulator and to increase their number by secondary ionization. Upon arrival at the insulator surface, the electrons and any negative ions which may have been formed by attachment to components of the quenching gas are collected in an image configuration forming a latent electrostatic image consisting of negative charges corresponding to elements or portions of the object which are relatively transparent to X-rays, and no charges or fewer charges corresponding to portions of the elements of the object which are opaque or relatively opaque to X-rays. The latent image is then made visible by development or by cathode ray tube display techniques.
One of the most significant limitations in the foregoing ionographic process is the photo-emissivity of the photoemissive layer. Since the electrons emitted are dependent upon the number of X-ray photons absorbed, the level of X-ray exposure has a significant impact on the results to be achieved in the ionographic process. One of the major advantages of the ionographic process as compared to normal X-ray techniques utilizing silver halide film is its ability to produce X-ray images with reduced X-ray exposure. As it known, it is most advantageous to be able to reduce the duration of X-ray exposure when individuals are being X-rayed. One way in which this can be accomplished in the inorgraphic process is by providing a photoemissive layer or material that has increased photoemissivity such that results equivalent to those obtainable with longer X-ray exposure periods can be achieved at shorter durations. In other words, it is highly desirable that sufficient electrons be produced by the photoemissive layer to achieve formation of a satisfactory latent image on an insulative substrate with reduced X-ray exposure. Heretofore, little effort was directed toward attempts at improving the photoemissivity of the photoemissive layer or the photoemissive layer utilized in the process of ionography. It was not heretofore known, for example, to employ as an ionographic photoemitter a material in the form of a thin film such as gold epitaxially grown on a substrate such as mica. However, it is known that gold can be epitaxially grown on mica. For example, see the paper by H. L. Chopra and L. C. Bobb entitled. "Electrical Conduction of Thin Epitaxially Grown Gold Films," Proceedings, International Conference on Single-Crystal Films, Blue Bell, Pennsylvania, May 1963. A related reference mentions lead deposition as well: "Thin Film Phenomena" by K. L. Chopra, McGraw-Hill, N.Y. 1966 (especially pages 225 and 236).